An asynchronous communication protocol for the receiving station to control the flow of data from the transmitting station. When the receiving station cannot continue to receive any more data, as when its buffer is full, it transmits an XOFF (DC3) control character that tells the sender to stop. When the receiving station is again able to process more data, it transmits an XON (DC1) control character that tells the sender to resume transmission. This protocol is also referred to as software handshaking.